The present invention relates to a method of spanning a high-molecular piezo-electric element across a frame body.
It is well known to provide a high-molecular-weight film having a strong piezo-electric property by polarizing under a high d.c. field a high-molecular-weight material, for example, polyvinylidene fluoride, polyvinyl fluoride, etc. Such high-molecular-weight piezo-electric films have numerous desirable characteristic features which could not be obtained with conventional inorganic piezo-electric substances. One of the greatest features of such elements is that they are soft and filmy and can easily be made to have a large surface area. Thus, such films are particularly applicable for use as vibrating films of sound transducers, such as head phones, speakers, etc., as well as various kinds of transducers, key boards, and so on.
When a high-molecular-weight piezo-electric film is used in any of the above-mentioned applications, it is usual to span said film across and adhere said film to a frame body. It is, however, very difficult to span a soft film on a frame body. For example, a high-molecular-weight piezo-electric film having a thickness of approximately 1-20 micrometers is used for vibrating films of head phones, but since such a thin film is extremely soft, it has no self-sustaining property and thereby produces flexure, wrinkles or the like in response to a slight external force. Accordingly, when the thin film is to be spanned on a frame body, wrinkles or local slackenings may easily be produced due to a slight unevenness of tension. However, it is necessary for sound transducers that the vibrating film be spanned uniformly; the above-mentioned wrinkles and slackenings deteriorate the performance of the transducer considerably.
In order to eliminate such a disadvantage, precautions are necessary when the high-molecular-weight piezo-electric film is spanned across a frame body. For example one technique includes, first clamping the periphery of the high-molecular-weight piezo-electric film with a jig that is larger than the frame body, adjusting the jig and film to obtain a uniform spanning without wrinkles or slackenings, and finally pasting the film on the frame body. Apparently such a process is not very efficient.